In optical communications systems, it is necessary to characterize the phase and amplitude of optical pulses as accurately as possible in order to predict and mitigate signal degradation. For example, in long distance wavelength-division multiplexed (WDM) systems optical signals may suffer degradation resulting from nonlinear effects such as self-phase modulation or cross-phase modulation.
As the need for information increases, so does the demand for higher speed and higher capacity communication systems. Higher speed communication systems require shorter optical pulses for transmission at higher bit rates (e.g., approximately 8 ps pulses for 40 Gb/s systems), and optical components with a picosecond response time or faster to process higher bit rate optical signals.
Techniques for the time-frequency analysis of the electrical field of a short optical pulse typically require a non-stationary filter element capable of modulating the amplitude and phase of the pulse on a time scale of the order of its duration. In the domain of femtosecond pulses, these techniques are generally realized using the nonlinear interaction of the short pulse to be characterized with one or several other short pulses in a quasi-instantaneous nonlinear medium. These nonlinear interactions require nonlinear optics which require fairly intense pulses. As such, these nonlinear techniques are impractical for low power applications such as telecommunication systems, which typically have peak powers as low as 0.1 mW or less. Various short pulse characterization techniques have been classified according to the way the information about the electric field is encoded in the experimental trace. For example, interferometric techniques directly measure the phase difference between a pair of optical frequencies in the spectral support of the pulse. Although various interferometric techniques have been demonstrated, Spectral Phase Interferometry for Direct Electric-field Reconstruction (SPIDER) is typically the most popular technique. SPIDER is a version of spectral shearing interferometry that relies on nonlinear optics. In SPIDER, a relative spectral shear between two replicas of a field under test is achieved by frequency mixing two time-delayed replicas of the pulse with a chirped pulse in a nonlinear optical crystal. However, the low efficiency of the nonlinear frequency conversion limits the sensitivity of SPIDER, as it does for all pulse characterization techniques based on nonlinear optics. Furthermore, the temporal support of the pulses that can be accurately characterized using SPIDER is also limited. SPIDER is also a free-space technology, and is accordingly impractical in the constraining environment of optical telecommunication. There is thus a need for more practical approaches to interferometric pulse characterization, for example using waveguide technology to implement interferometers and temporal phase modulators.